1. Field of the Invention
The present invention relates to gamma and X-ray backscatter imaging, and particularly to a collimator used in nondestructive testing, medical imaging, pipe inspection, general imaging and inspection and the like.
2. Description of the Related Art
Large insulated pipes are often found in gas producing plants for carrying both liquids and gas. The insulation around the pipes is necessary for maintaining relatively low liquid temperatures. Such insulated pipes are also commonly found in electric power plants, where the insulation is used to maintain a relatively high fluid temperature. The insulation layer on the pipes in these plants, as well as in various other industrial applications, is typically at least several centimeters thick, thus making it extremely difficult to inspect the pipe bodies for corrosion. Plant production must be stopped for interior visual inspection of the pipe walls, and removal of the outer insulation for exterior visual inspection not only requires a great deal of time and expense, but can be detrimental to the pipe itself. Ice forms on the exposed pipe surface for low temperature applications, accompanied by potentially dangerous increases of pressure in the interior, and heat is lost in high temperature applications. Additionally, such visual inspections of the pipe exterior will not indicate corrosion formed on the interior of the pipe. As noted above, conventional interior inspection would require a shutdown of the plant processes.
Although direct radiography allows for inspection of such pipes without the removal of the insulation layer, direct radiography has a number of drawbacks. As illustrated in FIG. 2, in conventional direct radiographic inspection, a radiation source 100 is positioned on one side of the object under inspection and radiographic film or an image plate is positioned opposite the source 100. In the specific application of insulated pipe inspection, a radiation source 100 emits radiation 102, which may be X-rays, gamma rays or the like, which pass through an insulated pipe, formed from a conventional pipe 106 carrying some sort of fluid 112 and surrounded by an outer annular insulation layer 104. A radiographic film or image plate 110 is placed beneath the pipe 106 for imaging corrosion 108 formed on the pipe 106.
The attenuation of X-ray and gamma ray radiation is very high in large bodies, such as in the exemplary insulated pipe of FIG. 2. If the object is very large, not enough radiation reaches the film or image plate 110 due to attenuation in the fluid 112 and in the metal wall (typically iron or iron-based materials) of the pipe 106. Additionally, as illustrated in FIG. 2, a relatively wide beam must be used, allowing for inspection of all sides of the pipe, which is often not possible for very large pipes or tanks. If a linear accelerator is used as the radiation source, such a wide beam is often impossible to produce. Further, due to the use of the single source, all sides of the pipe are imaged at the same time. This often creates confusion about the actual location of corrosion 108, since the image produced on the plate 110 is two-dimensional.
Gamma ray backscattering and X-ray fluorescence are known techniques for determining metal thickness, such as in measuring the thickness of corroded portions of metal bodies. In backscattered radiation imaging, a gamma ray beam is projected incident on the wall of the pipe. Its energy can be selected to be great enough that attenuation in the insulator is insignificant. As gamma rays or X-rays penetrate the pipe, the radiation undergoes attenuation, the radiation intensity decreasing exponentially with wall thickness. The magnitude of attenuation depends on the energy of the incident radiation and the nature of the material. Backscattering takes place from within layers of the wall by Compton interactions. The backscattered radiation undergoes higher attenuation in its path back to the detector or the film, since its energy is lower than that of the primary incident radiation. The radiation will, therefore, undergo double attenuation.
In X-ray fluorescence (XRF) imaging, the radiation incident interacts with the pipe material, followed by emission of XRF radiation. This type of X-ray is characteristic of wall materials. Most pipes and vessels of interest have walls made from iron or iron-based materials. The emitted X-rays have relatively small energies, typically around 7 keV. Additional detectors having high sensitivity for low energy radiation may be used if the first detector is not sensitive enough. It is generally preferable to use a radiation source that emits low energy in order to have a high level of reaction with the object materials. Because of the low energy of the XRF radiation, it is emitted from the surface of the object wall. Thus, it can image the outer surface of the object. This makes XRF desirable for insulated pipe inspection, since corrosion usually takes place in the outer surface of the pipe due to moisture trapped under the insulating layer.
In FIG. 3, a radioactive source 100 emits one or a few well-defined gamma rays. The radiation 102, which is incident on the pipe wall 106 (and passes through insulating layer 104), is collimated by a collimator 114. A portion of incident radiation 102 will backscatter due to Compton interactions, and a portion will also produce XRF radiation. The backscattered radiation 124 is measured by a gamma ray detector 118 (typically including a spectrometer, such as a Nal (Tl) scintillation detector), while the XRF radiation 120 is measured by a low energy X-ray detector 116, such as a CdTe, Si(Li) or HgI2 detector.
Backscattered radiation, measured at a fixed angle θ, and the XRF each give defined peaks when measured with energy analyzers, such as conventional multichannel analyzers. Counting windows can be selected to measure backscattered radiation peaks and XRF radiation. Single detectors, as illustrated in FIG. 3, are limited in their functionality, due to limitations in positioning, fixed degrees of angular measurement, and limited views of only portions of a pipe under inspection. More importantly, backscattered gamma radiation is not mono-directional. The backscattered rays are scattered in all directions, thus creating a fuzzy image when the radiation reaches the film or imaging plate. Thus, in order to select a parallel beam from the scattered radiation, it would be desirable to have a suitable collimator to use with the scattered radiation.
Several different approaches have been used for backscatter imaging. In point by point imaging, a narrow collimated beam is focused on a point on the surface of the object. Scattered radiation at a fixed angle is measured by a single detector, and the object and detector are moved in two dimensions. The scattered radiation from each point of the surface is used to construct an image. This type of imaging usually takes a relatively long time, though using a larger size detector or a stronger source can increase sensitivity and speed. In line by line imaging, a slit beam is incident on the surface and scattered radiation is measured by a linear detector array. In such a system, a large number of detectors is usually used. In plane by plane imaging a wide beam is incident on the object surface, and the scattered radiation is allowed to pass through a pinhole in an absorber and then fall onto a two dimensional image plate. In this arrangement, part of the incident radiation is obscured by the absorber itself. For applications such as corrosion measurements, which usually use high energy radiation, a thick absorber needs to be used in order to stop the radiation. Thus, a beam passing through a thick plate will not be sharp and image quality will be affected. Moreover, this arrangement would not provide time savings over point by point imaging because a long time is needed to collect enough radiation to form the image.
A flying spot system has also been used, in which a stationary horizontal slot collimator beam is intercepted by a rotating disc collimator that has radial slots. At the intersection of the line slot beam and the holes slots on the rotating disc, a narrow beam is defined. The system was successfully used for surface imaging of large objects but cannot be used for field imaging, such as nondestructive imaging, because it is relatively bulky, expensive and requires a large power supply. It also uses a low energy X-ray machine that cannot image thickness variations in thick wall objects, such as imaging corrosion in thick wall pipes.
Thus, a collimator for backscattered radiation imaging and a method of using the same solving the aforementioned problems is desired.